1. Field of the Invention
The present invention relates to polarimetric Doppler weather radar in general; and in particular, to a method and apparatus for transmitting and receiving generally elliptically polarized waves to allow measurement of linear orthogonal polarimetric variables, without the need to provide an apparatus with a high power, high frequency switch.
2. Description of the Prior Art
Polarimetric weather radars offer several advantages over conventional radars in estimating precipitation types and amounts. Foremost among them these advantages are the capability to discriminate between hail and rain, detect mixed phase precipitation, and much more accurately estimate rainfall. In one type of these radars, polarization is altered sequentially between linear vertical and linear horizontal to provide the following variables:
a) reflectivity factors at horizontal and vertical polarization; PA1 b) differential reflectivity for the two reflectivity factors; PA1 c) cumulative differential phase between the horizontally and vertically polarized echoes; PA1 d) correlation coefficient between the vertically and horizontally polarized echoes; and PA1 e) linear depolarization ratio. PA1 1) a long dwell time is needed to obtain estimates with acceptable errors; PA1 2) an active waveguide switch is needed to alternately select horizontally polarized and vertically polarized waves to the antenna feed (which switch is expensive and difficult to maintain); PA1 3) ground clutter filtering is compromised because it can be best done only on echoes of equal polarization; and, PA1 4) there is appreciable attenuation caused by the switch.
In addition, Doppler velocity and spectrum width can be obtained by suitably processing the horizontally and vertically polarized return signals.
Thus, in the prior art, a transmission system generates horizontal linearly polarized electromagnetic fields and vertical linearly polarized electromagnetic fields. The signal that generates these fields is supplied to a high power, fast switch, which sequentially switches between the two output waveguides so that a series of alternating horizontal and vertical linearly polarized fields are transmitted from the apparatus. Subsequently, the fields are received and processed in the known manner.
The standard procedure of alternating polarization has four drawbacks:
Currently, there is no polarimetric radar which simultaneously transmits horizontal and vertical polarization, and on reception separates the two linear orthogonal components.
In the paper Z.sub.DR Measurement Considerations for a Fast Scan Capability Radar by Sachidananda and Zrnic (Radio Science, Volume 20, Number 4, pp. 907-22, 1985), a system is proposed wherein linear polarization is transmitted at 45.degree. with respect to the vertical. Sachidananda and Zrnic suggest how to obtain differential reflectivity and phase with such an instrument. A similar proposal for differential reflectivity is provided in A Radar Configuration for Monopulse Differential Reflectivity Measurements, by R. G. Humphries, A. R. Holt, and P. L. Smith (Preprints, 25th International Conference on Radar Meteorology, American Meteorological Society, Boston, Mass., 1991, p. 622). The prior art scheme requires that two linear polarizations of equal amplitude and equal phase be combined at a feed node to produce a 45.degree. linear polarization. Although such an arrangement conceptually is simplistic, the requirements of having two linear polarizations of equal amplitude and phase would be very difficult to implement in practice. These two publications are incorporated, in their entirety, by reference.
A potential problem recognized by the Sachidananda and Zrnic is that with any kind of simultaneous transmission depolarization may occur by canted hydrometers along a propagation path; however, measurements made by National Severe Storms Laboratory (NSSL) present no evidence of such systematic canting over long propagation paths.
Other prior systems include that disclosed in U.S. Pat. No. 4,881,077 to Jehle, et al., U.S. Pat. No. 5,086,301 to English et al., U.S. Pat. No. 4,849,762 to Barnes, U.S. Pat. No. 5,247,303 to Cornelius et al., U.S. Pat. No. 5,038,150 to Bains, U.S. Pat. No. 4,742,354 to Wen et al. and U.S. Pat. No. 5,175,551 to Rubin, the entire disclosures of which are incorporated by reference, in their entirety.